diff --git a/plugins/kolab_core/rcube_kolab.php b/plugins/kolab_core/rcube_kolab.php
index 92ec41fa..e01de1cf 100644
--- a/plugins/kolab_core/rcube_kolab.php
+++ b/plugins/kolab_core/rcube_kolab.php
@@ -51,8 +51,8 @@ class rcube_kolab
         }
 
         // Re-set LDAP/IMAP host config
-        $conf['kolab']['ldap'] += array('server' => 'ldap://' . $_SESSION['imap_host'] . ':389');
-        $conf['kolab']['imap'] += array('server' => $_SESSION['imap_host'], 'port' => $_SESSION['imap_port']);
+        $conf['kolab']['ldap'] = array('server' => 'ldap://' . $_SESSION['imap_host'] . ':389');
+        $conf['kolab']['imap'] = array('server' => $_SESSION['imap_host'], 'port' => $_SESSION['imap_port']);
 
         // pass the current IMAP authentication credentials to the Horde auth system
         self::$horde_auth = Auth::singleton('kolab');